Catalytic growth of single-wall carbon nanotubes from metal particles

ABSTRACT

Single-walled carbon nanotubes have been synthesized by the catalytic decomposition of both carbon monoxide and ethylene over a supported metal catalyst known to produce larger multi-walled nanotubes. Under certain conditions, there is no termination of nanotube growth, and production appears to be limited only by the diffusion of reactant gas through the product nanotube mat that covers the catalyst The present invention concerns a catalyst-substrate system which promotes the growth of nanotubes that are predominantly single-walled tubes in a specific size range, rather than the large irregular-sized multi-walled carbon fibrils that are known to grow from supported catalysts. With development of the supported catalyst system to provide an effective means for production of single-wall nanotubes, and further development of the catalyst geometry to overcome the diffusion limitation, the present invention will allow bulk catalytic production of predominantly single-wall carbon nanotubes from metal catalysts located on a catalyst supporting surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to methods of producing single-wallcarbon nanotubes, and to catalysts for use in such methods.

2. Description of Related Art

Fullerenes are closed-cage molecules composed entirely of sp2-hybridizedcarbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) werefirst identified as closed spheroidal cages produced by condensationfrom vaporized carbon.

Fullerene tubes are produced in carbon deposits on the cathode in carbonarc methods of producing spheroidal fullerenes from vaporized carbon.Ebbesen et al. (Ebbesen I), “Large-Scale Synthesis Of Carbon Nanotubes,”Nature, Vol. 358, p. 220 (Jul. 16, 1992) and Ebbesen et al., (EbbesenII), “Carbon Nanotubes,” Annual Review of Materials Science, Vol. 24, p.235 (1994). Such tubes are referred to herein as carbon nanotubes. Manyof the carbon nanotubes made by these processes were multi-wallnanotubes, i.e., the carbon nanotubes resembled concentric cylinders.Carbon nanotubes having multiple walls have been described in the priorart. Ebbesen II; Iijima et al., “Helical Microtubules Of GraphiticCarbon,” Nature, Vol. 354, p. 56 (Nov. 7, 1991).

Another known way to synthesize nanotubes is by catalytic decompositionof a carbon-containing gas by nanometer-scale metal particles supportedon a substrate. The carbon feedstock molecules decompose on the particlesurface, and the resulting carbon atoms then diffuse through theparticle and precipitate as part of nanotubes growing from one side ofthe particle. This procedure typically produces imperfect multi-wallednanotubes in high yield. See C. E. Snyder et al., International PatentApplication WO 89/07163 (1989), hereby incorporated by reference in itsentirety. Its advantage is that it is relatively simple and can bescaled to produce nanotubes by the kilogram.

Single-wall carbon nanotubes have been made in a DC arc dischargeapparatus of the type used in fullerene production by simultaneouslyevaporating carbon and a small percentage of Group VIII transition metalfrom the anode of the arc discharge apparatus. See Iijima et al.,“Single-Shell Carbon Nanotubes of 1 nm Diameter,” Nature, Vol. 363, p.603 (1993); Bethune et al., “Cobalt Catalyzed Growth of Carbon Nanotubeswith Single Atomic Layer Walls,” Nature, Vol. 63, p. 605 (1993); Ajayanet al., “Growth Morphologies During Cobalt Catalyzed Single-Shell CarbonNanotube Synthesis,” Chem. Phys. Lett., Vol. 215, p. 509 (1993); Zhou etal., “Single-Walled Carbon Nanotubes Growing Radially From YC2Particles,” Appl. Phys. Lett., Vol. 65, p. 1593 (1994); Seraphin et al.,“Single-Walled Tubes and Encapsulation of Nanocrystals Into CarbonClusters,” Electrochem. Soc., Vol. 142, p. 290 (1995); Saito et al.,“Carbon Nanocapsules Encaging Metals and Carbides,” J. Phys. Chem.Solids, Vol. 54, p. 1849 (1993); Saito et al., “Extrusion of Single-WallCarbon Nanotubes Via Formation of Small Particles Condensed Near anEvaporation Source,” Chem. Phys. Lett., Vol. 236, p. 419 (1995). It isalso known that the use of mixtures of such transition metals cansignificantly enhance the yield of single-wall carbon nanotubes in thearc discharge apparatus. See Lambert et al., “Improving ConditionsToward Isolating Single-Shell Carbon Nanotubes,” Chem. Phys. Lett., Vol.226, p. 364 (1994). While this arc discharge process can producesingle-wall nanotubes, the yield of nanotubes is low and the tubesexhibit significant variations in structure and size between individualtubes in the mixture. Individual carbon nanotubes are difficult toseparate from the other reaction products and purify.

High quality single-wall carbon nanotubes have also been generated byarc evaporation of a graphite rod doped with Y and Ni. See C. Journet etal., Nature 388 (1997) 756, hereby incorporated by reference in itsentirety. These techniques allow production of only gram quantities ofsingle-wall carbon nanotubes.

An improved method of producing single-wall nanotubes is described inU.S. Ser. No. 08/687,665, entitled “Ropes of Single-Walled CarbonNanotubes” incorporated herein by reference in its entirety. This methoduses, inter alia, laser vaporization of a graphite substrate doped withtransition metal atoms, preferably wall nanotubes on the catalystparticles of less than 2-nanometer diameter but is formed into graphiticlayers that encapsulate the larger catalyst particles, deactivating themas catalysts. Catalyst particles of greater than about 2 nanometers indiameter are more likely to form multiwall nanotubes, and since they arerendered ineffective by the process, the only remaining active catalystparticles support growth of primarily single-wall nanotubes. In apreferred embodiment, the method of this invention provides fortreatment of supported catalyst material to deactivate catalystparticles that do not support growth of the desired nanotube types, withsubsequent change of the feedstock composition or density to accelerategrowth of the desired form of single-wall nanotubes. The method of thisinvention is capable of producing-material that is >50% SWNT, moretypically >90% SWNT, or even >99% SWNT.

This invention also provides a catalyst/support system structured sothat access of the feedstock gas to the catalyst is enhanced by thatstructure. Preferably, the catalyst is deposited so that there is cleardistance between catalyst locations by dispersion of small catalystparticles on the substrate surface or other methods of deposition knownto those skilled in the art.

The production of high quality single-wall carbon nanotubes, in somecases including double-wall carbon nanotubes, in yields much larger thanpreviously achieved by catalytic decomposition of carbon-containingprecursor gases is disclosed. The nanotubes formed are connected to andgrow away from the catalyst particles affixed to the catalyst supportsurface. If the growth time is short, the tubes can be only a fractionof one micron long, but if the growth time is prolonged, single-wallcarbon nanotubes in this invention can grow continuously to arbitrarylengths. The present invention demonstrates a means for nucleating andgrowing nanotubes only from the smallest of the supported catalystparticles, which produce single-wall carbon nanotubes, whiledeactivating the larger particles so that no multi-walled nanotubes areproduced. This allows the growth exclusively of single-wall carbonnanotubes from catalyst systems previously thought to produce onlylarger diameter multi-walled nanotubes.

According to one embodiment of the present invention, a process forproducing single wall carbon nanotubes is disclosed. The processcomprises the nanotubes) unless the process is carried out with excesshydrocarbon feedstock. The product of a typical process for makingmixtures containing single-wall carbon nanotubes is a tangled felt,which can include deposits of amorphous carbon, graphite, metalcompounds (e.g., oxides), spherical fullerenes, catalyst particles(often coated with carbon or fullerenes) and possibly multi-wall carbonnanotubes. The single-wall carbon nanotubes may be aggregated in “ropes”or bundles of essentially parallel nanotubes.

Nanotubes prepared using the catalytic method of this invention tend tobe less contaminated with pyrolytic or amorphous carbon than nanotubesprepared by prior art methods. Furthermore, by using a catalyst with anarrow size distribution, the nanotubes produced consequently have anarrow size distribution. This will minimize the need forpost-production activities to clean up the nanotube preparation. To theextent that the nanotube product contains pyrolytic carbon which needsto be removed, various procedures are available to the skilled artisanfor cleaning up the product. Suitable processes for purifying carbonnanotubes prepared according to this invention include the processesdescribed in International Patent Publication WO 98/39250.

According to the invention, predominantly single-wall carbon nanotubes,with a portion of double-wall carbon nanotubes under some conditions,are produced with diameters in the range from about 0.5 to about 3 nm.Typically, no 5 to 20 nm diameter multi-walled nanotubes are produced bysupported catalyst particles. The key difference responsible for theseeffects is that the growth reaction rate is limited by the supply ofcarbon to the catalyst particles, whereas the multi-walled nanotubegrowth is thought to be limited by the diffusion of carbon through thecatalyst particles.

The single-wall nanotubes of the present invention may have lengthsexceeding one micron. The length may be controlled by lengthening orshortening the amount of time the catalyst is exposed to the feedstockgas at an appropriate temperature and pressure. In one embodiment, underproper conditions the single-wall nanotubes can grow continuously to anarbitrary length.

Single-wall nanotubes formed in the present invention are observed toform into organized bundles or “ropes” as they grow from catalystparticles in close proximity to each other. Examples of this behaviorare shown in FIG. 4 b. Such ropes of SWNT may be removed from thesupported catalyst for subsequent processing and/or utilization, or theymay be used “as is” while still attached to the catalyst particle. SWNTprepared according to this invention using a supported catalyst withwidely dispersed catalytic particles may be recovered prior toaggregation of the individual nanotubes. These nanotubes may becollected in the form of a mat or felt with random orientation in twodimensions or individually for particular uses.

Using the product nanotubes

Carbon nanotubes, and in particular the single-wall carbon nanotubes ofthis invention, are useful for making electrical connectors in microdevices such as integrated circuits or in semiconductor chips used incomputers because of the electrical conductivity and small size of thecarbon nanotube. This invention provides a means of establishing acarbon nanotube directly in contact with a surface, but extending awayfrom that surface. This occurs naturally in the present invention as thetube is grown from a catalyst particle in contact with the surface of alarger object (the catalyst support). This invention's provision of asimple means for creating structures that comprise a support surfacewith one or more nanotubes attached and extending away from that surfaceis particularly useful in known applications of nanotubes as probes inscanning tunneling microscopes (STM) and atomic force microscopes (AFM)and as field emitters of electrons for electronic applications. Thecarbon nanotubes are useful as antennas at optical frequencies, and asprobes for scanning probe microscopy such as are used in scanningtunneling microscopes (STM) and atomic force microscopes (AFM). Thecarbon nanotubes are also useful as supports for catalysts used inindustrial and chemical processes such as hydrogenation, reforming andcracking catalysts. The nanotubes may be used, singularly or inmultiples, in power transmission cables, in solar cells, in batteries,as antennas, as molecular electronics, as probes and manipulators, andin composites.

EXAMPLE

In order to facilitate a more complete understanding of the invention,an Example is provided below. However, the scope of the invention is notlimited to specific embodiments disclosed in this Example, which is forpurposes of illustration only.

1. Preparation

Single wall carbon nanotubes may be grown by passing carbon-containinggases (CO or C₂H₄) at elevated temperatures over nanometer-size metalparticles supported on larger (10-20 nm) alumina particles. Twodifferent metal catalysts may be used, one containing pure Mo, the othercontaining Fe and Mo. The ratio of FE to Mo may be 9:1. Both catalystswere made using a method known in the art.

For each growth experiment, a quartz boat containing a carefully weighedamount (typically 20 mg) of the catalyst powder was placed in the centerof a 1 inch quartz tube furnace. The system was purged with Ar, thenheated under flowing reactant gases to an elevated temperature for acontrolled time. The resulting catalyst material, which now alsocontains reaction products dominated by single-wall carbon nanotubes,was removed from the boat and weighed again. The yield is defined as themass increase divided by the original catalyst mass. Samples wereprepared for TEM imaging by sonicating this material in methanol anddrop-drying the resulting suspension onto TEM grids.

2. Production of single-wall carbon nanotubes

The production of single-wall carbon nanotubes by the disproportionationof CO over alumina-supported Mo particles is greatly improved. Thecatalyst is 34:1 alumina:Mo by mass. The reaction is carried out at 850°C. under a flow of 1200 sccm of CO at 900 Torr. The resulting material,which consists of single-wall carbon nanotube very monodisperse indiameter (0.8 to 0.9 nm), is shown in FIG. 1. Particles of the fumedalumina support, 10 to 20 nm in size, are also visible in this andsubsequent TEM images. The yield of nanotubes is plotted as a functionof reaction time in FIG. 2. The yield continues to increase even forvery long reaction times.

CO also forms nanotubes with a second catalyst. The second catalyst isprepared with 90:9:1 alumina:Fe:Mo by mass. The reaction, when carriedout exactly as described above for the alumina:Mo catalyst, yieldsnanotubes of a wider diameter distribution, 0.5 to 3 nm, withsingle-wall carbon nanotubes and some double-wall carbon nanotubes. Arepresentative TEM image is shown in FIG. 3. For this catalyst, theyield increases with time initially, but is limited to about 40% afterone hour of exposure. No additional mass increase is observed even formuch longer exposures (up to 20 hours).

Single-wall carbon nanotubes from C₂H₄ have been grown using thistechnique. The 90:9:1 alumina:Fe:Mo catalyst is first reduced byexposing the catalyst to 1000 sccm Ar and 0.33 sccm H₂ at 800° C. for 30minutes. The growth reaction then proceeds at the reaction temperatureby adding 0.66 sccm C₂H₄ to the gas flow. The resulting product isnanotube bundles containing single-wall carbon nanotubes and double-wallcarbon nanotubes, shown in FIGS. 4 and 5. One hundred nanotube crosssections were observed at several reaction temperatures to count therelative number of single- to double-walled nanotubes. The amount ofdouble-wall carbon nanotubes increases from 30% at 700° C. to 70% at850° C. Outer diameters of the individual tubes in a bundle range from0.5 to 3 nm. There appears to be no correlation between outer diameterand number of walls, other than that the smallest nanotubes (<1 nmdiameter) are never double-walled.

The mass yield of nanotubes increases at a similar rate for reactiontemperatures from 700° C. to 850° C., but the termination is temperaturedependent. For reactions at 850° C., the yield increases until itreaches 7%, at which point the growth terminates. As the reactiontemperature is lowered, the yield reaches higher levels before growthtermination. At 700° C., the growth does not terminate, but its ratedecreases as shown in FIG. 2.

The present invention demonstrates the ability to grow nanotubes bycatalytic decomposition of C₂H₄ and CO only from the small particles ina supported catalyst system, leading to the growth of single-wall carbonnanotubes and deactivation of multi-walled nanotube growth byencapsulation of larger particles. For certain conditions, nanotubes canbe grown to arbitrary length, but become limited by the diffusion ofreactants to the catalyst particles. This problem has been solved forthe production of multi-walled nanotubes from this catalyst by usingflat alumina flakes, as opposed to fumed alumina particles, so that thenanotubes grow aligned in large bundles, keeping their growing endsexposed to the gaseous feedstock. Similar modifications to the currenttechnique may allow the bulk production of single-wall carbon nanotubes.

While the invention has been described in connection with preferredembodiments, it will be understood by those skilled in the art thatother variations and modifications of the preferred embodimentsdescribed above may be made without departing from the scope of theinvention. Other embodiments will be apparent to those skilled in theart from a consideration of the specification or practice of theinvention disclosed herein. It is intended that the specification isconsidered as exemplary only, with the true scope and spirit of theinvention being indicated by the following claims.

1. A process for producing single-wall carbon nanotubes comprising:providing an active nanoscale particulate transition metal catalystsupported on an inert surface in a reaction zone maintained at anelevated temperature; supplying to said reaction zone a gaseouscarbon-containing compound under conditions that inactivate catalystparticles having a diameter large enough to catalyze the production ofmulti-wall carbon nanotubes; and contacting said gaseouscarbon-containing compound in the reaction zone with small diameteractive supported catalyst particles, wherein small diameter activesupported catalyst particles catalyze the production of single-wallcarbon nanotubes.
 2. The method of claim 1, wherein individualsingle-wall carbon nanotubes grow away from the catalyst supportsurface.
 3. The method of claim 1, wherein nanotubes grow away from thesupport surface in bundles of parallel tubes.
 4. The method of claim 1,wherein 30% of the nanotubes in said bundles are single-wall carbonnanotubes.
 5. The method of claim 1, wherein 70% of the nanotubes insaid bundles are single-wall carbon nanotubes.
 6. The method of claim 1,wherein an outer diameter of the nanotubes in said bundle range fromabout 0.5 to about 3 nm.
 7. The method of claim 1, wherein inactivationof the catalyst particles that support multi-wall nanotube growth isachieved by limiting the carbon supply to the heated catalyst particles.8. The method of claim 1, wherein said at least one carbon containinggas is selected from the group consisting of CO, C₂H₄, and combinationsthereof.
 9. The method of claim 1, wherein said metal particle isselected from the group consisting of Group VIB transition metals,chromium (Cr), molybdenum (Mo), tungsten (W) and Group VIIIB transitionmetals, e.g., iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru),rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum(Pt) and mixtures comprising any of these.
 10. The method of claim 9,wherein said metal particle comprises at least two of the listed metals.11. The method of claim 1, wherein a lower concentration of carbon insaid at least one carbon containing gas allows each carbon atom in saidsingle-wall carbon nanotube sufficient time to anneal to its lowestenergetic configuration.
 12. The method of claim 1, wherein said supportcomprises flat alumina flakes.
 13. The method of claim 1, wherein saidsolid catalyst support is porous, permitting passage of saidcarbon-containing gas therethrough.
 14. The method of claim 1 or claim13, wherein said metal particles were distributed on said support toenhance access of said carbon-containing gas to growing single-wallnanotubes.
 15. The method of claim 14, where said metal particles aredistributed to provide clear space between particles.